Robotic steerable catheter systems typically include a flexible catheter shaft having an articulation section at a distal tip. These systems are designed to facilitate access to distal target sites in the human anatomy and require simultaneous articulation of the distal tip with continued insertion or retraction of the catheter. Pullwire based articulating catheters typically have pullwires passing through the shaft, and each pullwire is anchored to a fixed location around the distal tip. Each pullwire is then selectively tensioned to articulate the tip in various directions. As such, the catheter shaft should be laterally flexible to follow the curvature in the anatomy, but axially rigid to resist the high axial loads being applied to articulate the catheter tip.
Increasing the lateral flexibility of the catheter, however, introduces catheter navigation problems that may not otherwise occur when the catheter is laterally stiff. For example, many steerable catheters have a multitude of free floating pullwires (e.g., four pullwires), circumferentially spaced in the wall of the catheter and attached to a control ring embedded in the distal end of the catheter. If four pullwires are provided, the pullwires may be orthogonally spaced from each other. Each of these pullwires is offset from the center axis of the catheter, so that when a wire is tensioned to steer the catheter's distal tip under ideal conditions, the resulting bending moment causes the distal tip to articulate in the direction of the pullwire that is tensioned. However, the compressive forces from the tensioned pullwire on the relatively flexible catheter shaft also cause the shaft to compress and/or to experience other undesired effects.
For example, flexible shafts adapt to the shape of the anatomy as they track through it. This results in a curved shaft. The curvature of the catheter shaft may make the articulation performance of the catheter unrepeatable and inconsistent. In particular, because the pullwires are offset from the neutral axis of the catheter shaft, bending the catheter shaft causes the pullwires on the outside of the curve to tighten while the pullwires on the inside of the curve slacken. As a result, the amount of tension that should be applied to the pullwires in order to effect the desired articulation of the distal tip varies in accordance with the amount of curvature applied to the catheter shaft.
Referring to FIGS. 1A and 1B, a prior art catheter 10 with an articulating distal portion 11 (or “distal tip”) is shown, to illustrate another example of the challenges faced when articulating a catheter in a body. As illustrated, one challenge is that the articulated distal tip 11, when bent, tends to align its curvature with the curvature of the shaft of the catheter 10. In particular, as shown in FIG. 1B, operating or tensioning a pullwire 14 on the outside edge of a bend may cause the catheter 10 to rotate or twist. This rotation or twist phenomenon is known as “curve alignment,” because the distal tip 11 and shaft of the catheter 10 tend to rotate until the tensioned pullwire 14 is on the inside of the bend, and the curve in the distal tip 11 is aligned with the curvature in the shaft. That is, when the proximal shaft section of the catheter 10 is curved (as it tracks through curved anatomy), and the distal tip 11 is commanded to articulate, the curvature in the shaft can impact the articulation performance of the distal tip 11.
In FIG. 1A, the pullwire 12 that is pulled happens to be on the inside of the bend of the catheter shaft 10, and the distal tip 11 articulates to the left as intended. However, if it is desired to bend the distal tip 11 in a direction that is not aligned with the curvature of the proximal portion of the catheter 10, (e.g., if it is desired to bend the distal tip 11 to the right, as shown by the dotted distal tip outline in FIG. 1B), the pullwire 14 on the outside of the bend is pulled. A torsional load (T) is applied to the shaft as tension increases on the pullwire 14 on the outside of the bend. This torsional load rotates the shaft until the pulled pullwire 14 is on the inside of the bend. As shown in FIG. 1B, the initial position of the outer pullwire 14 is depicted by a dashed line, and the rotated position following application of the torsional load is depicted by the solid line. In effect, the tensioned pullwire 14 on the outside of the bend takes the path of least resistance, which may often rotate the shaft to the inside of the bend (as shown by the thick, solid-tipped arrows in FIG. 1B), rather than articulating the distal tip 11 as the user intends. This results in the distal tip 11 pointing to the left, as shown in the solid-line version of the distal tip 11, even though the user wanted to bend the distal tip 11 to the right, as shown in the dotted-line version. This unintentional rotation of the shaft causes instability of the catheter distal tip 11 and prevents the physician from being able to articulate the distal tip 11 to the right. In other words, no matter which direction the catheter distal tip 11 is intended to be bent, it may ultimately bend in the direction of the proximal curve. This phenomenon is known as curve alignment, because the pullwire 14 that is under tension puts a compressive force on both the proximal and distal sections of the catheter 10 causing both the proximal and distal curvatures to align in the same direction in order to achieve the lowest energy state.
The operator may attempt to roll the entire catheter 10 from the proximal end in order to place the articulated distal tip in the desired direction. However, this moves the tensioned inside pullwire 14 to the outside of the proximal bend, causing further tensioning of the pullwire 14. This increased tension on the pullwire 14 on the outside of the bend can cause an unstable position. The catheter shaft 10 wants to return to a lower energy state and may do so by quickly whipping around to get the tensioned pullwire 14 back to the inside of the bend. In a multi-direction catheter, the operator may attempt to pull a different pullwire to try to bend the distal tip to the right, but as soon as the tension is built up on that wire, it also wants to spin the distal tip around and return to the inside of the bend. Continued attempts to try to find a pullwire to articulate the distal tip against the direction of curvature of the catheter shaft may lead to rotation or windup of the catheter shaft. This stored energy in the shaft can lead to whipping of the catheter shaft to return to a lower energy state and may injure the patient.
FIGS. 2A and 2B illustrate another example of the challenges faced when articulating a flexible catheter 10. When performing a steering maneuver with a flexible catheter 10, the tension on the pullwire(s) causes axial compression on the catheter shaft, which bends the distal tip 11 of the catheter 10. This axial compression may cause undesired lateral deflection in flexible catheter shafts, thereby rendering the catheter mechanically unstable. FIGS. 2A and 2B illustrate how prior art flexible instruments exhibit unwanted lateral shaft deflection when one or more pullwires are pulled. In these figures, the pullwires run through the wall of the catheter shaft. An example of ideal articulation performance is shown in FIG. 2B. If the shaft is made of stiff materials, then the catheter distal tip 11 is more likely to exhibit ideal articulation performance. If the catheter shaft is made of more flexible, trackable materials, then the catheter 10 is more likely to bend as shown in FIG. 2A, with bending occurring not only in the distal tip 11, but also along a length of the catheter shaft. The shaft of the catheter is being muscled by the pullwires and experiencing unwanted lateral deflection.
The additional lateral deflection of the shaft of the catheter 10 may be undesirable, because it may unintentionally force the catheter against the anatomy. This has the potential for injury and distracts the operator, because he or she must constantly monitor what the shaft is doing. If the shaft is in a constrained position within the arteries, such as passing over the iliac bifurcation, the arterial rigidity may stop the shaft from being muscled by the pullwires. But alternatively, the catheter shaft may be in a more flexible artery, such as the splenic artery, where the catheter may damage or distort the shape of the artery.
Referring to FIGS. 3A-3C, if a catheter 10 is in a large artery or open chamber, such as the aorta or heart, the catheter 10 may have space to deflect. This creates an additional problem, because the more space the catheter shaft has to deflect, the greater the impact on the amount of catheter tip articulation. For example, FIGS. 3A-3C show the catheter 10 in three different configurations. In FIG. 3A, both the proximal shaft and the distal articulation tip 11 are straight. In FIG. 3B, a pullwire has been pulled a distance x, and the articulation tip 11 has bent 90 degrees. The proximal shaft has not bent. This may occur, for example, when the shaft is constrained by the anatomy. In FIG. 3C, the pullwire has been tensioned an equal amount as in FIG. 3B, but the shaft has also compressed. Therefore, in FIG. 3C, some of the pullwire displacement has been “used up” to compress the shaft, and hence there is less compression of the articulation tip 11. As a result, the articulation tip 11 only bends approximately 80 degrees in FIG. 3C. It would be desirable to isolate bending to the distal articulation tip 11, to aid in predictability and controllability. In other words, an ideal catheter instrument would have a distal articulation tip 11 that bends as commanded and is not dependent on the anatomical path or the stiffness of the vasculature.
Undesirable lateral motion related to muscling and undesirable rotational motion related to curve alignment both result from the same forces associated with pullwire tensioning. Each of these mechanical challenges contributes to the instability and poor control of the catheter tip, as well as decreased catheter tracking performance. Some steerable catheters overcome these problems and resist compressive and torsional forces by increasing the axial stiffness of the entire catheter shaft (e.g., by varying wall thickness, material durometer, and/or braid configuration), or alternatively by incorporating axially stiff members within the catheter shaft to take the axial load. But these changes also laterally stiffen the catheter shaft, making it less maneuverable, and thereby causing new difficulties in tracking the catheter through the vasculature of the patient. Therefore, the catheter designer is forced to compromise between articulation performance and shaft tracking performance.
Another design intended to overcome the problems of muscling and curve alignment involves locating all the pullwires in the shaft close to the neutral axis, as described in U.S. Pat. No. 8,894,610. This is known as the “unirail design” for a catheter. While the unirail design locates all pullwires in one location, it is impossible to locate all pullwires exactly on the neutral axis, so the catheter continues to experience some slight unwanted shaft curvature. Catheter designers typically need to design some lateral stiffness into the catheter shaft, to try to minimize this unwanted curvature. Therefore, the shaft of the unirail catheter cannot be designed with very low lateral stiffness.
Another strategy is to spiral the pullwires around the circumference of the catheter shaft, as described in U.S. patent application Ser. No. 14/542,373 (U.S. Patent App. Pub. No. 2015/0164594). This is known as the helical design and can be used to balance loads in the catheter shaft. However, continuously spiraling the pullwires leads to increased friction in the catheter system, and so there is still a tradeoff between shaft flexibility and articulation performance.
Other steerable catheters overcome this problem by using free floating coil pipes in the wall of the catheter to respectively house the pullwires, thereby isolating the articulation loads from the catheter shaft. (Embodiments and details are described in U.S. patent application Ser. No. 13/173,994, entitled “Steerable Catheter,” (U.S. Patent App. Pub. No. 2012/0071822), which is expressly incorporated herein by reference in its entirety.) However, the use of coil pipes adds to the cost of the catheter and takes up more space in the shaft, resulting in a thicker catheter wall. Such a design is not appropriate for catheters with small outer diameters intended for use in narrow vasculature.
Pullwire-based steerable catheters typically incorporate the steering pullwires into the walls of the catheters, and the catheters must be designed to accommodate the thickness and arrangement of the pullwires. Referring to FIGS. 4A-4C, various examples of pullwire-based steerable catheters 10A-10C are provided, each including steering pullwires 12A-12C in the wall of the respective catheter. The diameter of the steering pullwires 12A-12C usually determines the wall thickness that can be achieved. For example, in the embodiment illustrated in FIG. 4A, there are four small pullwires 12A evenly spaced around the circumference of the catheter 10A, whereas in FIG. 4C, there are three larger diameter pullwires 12C equally spaced around the circumference of the catheter 10C. The embodiment in FIG. 4A has a thinner wall, due to the smaller diameter of the pullwires 12A. Thinner walls are preferable, because, as shown, they allow for a larger inner diameter (ID) for a given outer diameter (OD), or a smaller OD for a given ID. In other words, thin walls allow for the smallest OD:ID ratio. Advantageously, larger inner diameters allow for delivery of a broader range of tools. Smaller outer diameters allow for access to narrower blood vessels, thereby increasing the number of procedures that can be performed with steerable catheters. Smaller ODs also allow for smaller incisions in patients and hence, faster recovery times.
One barrier to achieving a small OD:ID ratio is the diameter of the pullwires. For example, the relatively small pullwires 12A in FIG. 4A have less tensile strength than the pullwires of FIGS. 4B and 4C, and this can limit the articulation force that can be applied to bend the distal articulation tip. The embodiment in FIG. 4B is an alternative option, which uses larger pullwires 12B while maintaining a larger ID. Here, the OD and ID of the catheter 10B are not concentric. There is only one pullwire 12B, so the wall thickness is thinner in the area opposite the pullwire 12B to maximize the inner lumen. This catheter embodiment 10B, however, has a reduced degree of freedom (i.e., less maneuverability) at the distal tip. Accordingly, each design has significant tradeoffs.
Thus, although a number of innovations have been made, major unresolved challenges remain when using pullwires to articulate the distal tip of a flexible catheter. It would, therefore, be desirable to have improved steerable catheters, designed to particularly address at least some of the challenges described above. Ideally, such improved catheters would have a desired combination of stiffness, flexibility, and ease of articulation. Also ideally, the catheters would have a desirable inner diameter and outer diameter to make them suitable for passing instruments and for advancing through small incisions and vasculature. At least some of these objectives will be addressed by the embodiments described herein.